<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .search{
            position: relative;
            width: 178px;
            margin: 100px;  
            border: #000 2px solid;                  
        }
        .con {
            display: none;
            position: absolute;
            top: -40px;
            width: 170px;
            border: 1px solid rgba(0,0,0,0.8);
            box-shadow: 0 2px 4xp rgba(0,0,0,0.8);
            padding: 5px 0;
            font-size: 18px;
            line-height: 20px;
            color: #333;                    
        }
        .con::before{
            content: '';
            width:0;
            height:0;
            position: absolute;
            top:28px;
            left:18px;
            border: 8px solid #000;
            border-style: solid dashed dashed;
            border-color: #fff transparent transparent     
        }
    </style>
</head>
<body>
    <div class='search'>
        <div class='con'>123</div>
        <input type='text' placeholder='请输入您的快递单号' class="jd"></input>
    </div>
    <script>
        var con = document.querySelector('.con')
        var jd_input = document.querySelector(".jd")
        jd_input.addEventListener('keyup',function(e){
            if(this.value == ''){
                con.style.display = 'none'            
            }else{
                con.style.display = 'block'
                con.innerText = this.value;              
            }                              
        })
    </script>
</body>
</html>